The present invention relates, in general, to ionization detectors and is more particularly concerned with a device for detecting fires which preferably employs a beta source, although the teachings of this invention may also be applicable to use with other types of sources.
There are numerous different types of ionization fire alarm devices which are know. These detectors typically comprise one or two chambers and one or two radio-active sources. These devices operate on the basic principal of a change in the ionization current within the chamber upon detection of products of combustion and aerosols in the atmosphere where the detector is located.
Most of these detectors, including virtually all commercial detectors, employ an alpha source such as Americium 241. While these sensors have gained acceptance and are widely used in fire detection systems, it is well known that alpha particles are very much more hazardous than beta particles. It has been argued that normally the radiation is trapped within the ionization chamber and thus there is no problem. However, there are circumstances which have occurred wherein a detector using alpha particles has become hazardous. For example, situations have arisen after a fire where detectors have been lost in the rubble thus making disposal of the rubble a problem.
Accordingly, to make a safer device, it would be desirable to construct a detector using a low activity beta radiation source. Even some of the prior art patents such as U.S. Pat. Nos. 3,573,777; 3,271,756; 3,295,121; and 3,560,737 have mentioned the beta source as a possible radiation source for ionization detectors. However, generally speaking there is no detector currently available that uses a beta radiation source. There are many factors that may account for this lack of a use of the beta source. Generally, beta sources which have been considered were of the high activity type and thus were not suitable for constructing compact detectors. Other beta sources, such as Tritium, have a short half-life and present mechanical problems, such as migration. Therefore, these detectors were not suitable for use in ionization detection. In accordance with this invention preferably a low activity beta source is used such as nickel 63.
A further problem in the prior art with the use of beta sources is the extremely low ionization current that is available with these detectors. This usually results in difficulties with the associated electronic circuitry as well as producing problems regarding detection of extraneous noise signals. In accordance with this invention, the design of the chamber structure and the choice of the circuitry greatly reduce the problem of the low ionization current.
Still another problem associated with known ionization detectors is that, because the detectors may be used in different environments, it is difficult to produce a detector that will operate suitably in all of these environments without requiring adjustment in the field. In the past, many of these detectors were subject to humidity changes and air density changes which affected the sensitivity of the detector. Also, another problem with known detectors using radioactive sources is the tolerance of the source itself. While dimensions within the chamber can be held to a very close tolerance, radiation activity differs from source to source.
For example, U.S. Pat. Nos. 3,295,121 and 3,271,756 reveal a means for adjusting voltages at the ionization chamber output. However, these means rely on the alteration of the chamber geometry or the adjustmant of distance electrodes. This is a complex mechanical adjustment and will not give the degree of control as that provided by the adjustment means of the present invention. With the adjustable electrode of the present invention, detectors may be constructed with wide variations in sources from one detector to another.
Accordingly, it is one object of the present invention to provide a safe and reliable apparatus for detecting products of combustion and aerosols in a gas or typically the atmosphere.
A further object of the present invention is to provide a detector which is easy to produce and easy to adjust for optimum performance.
Another object of the present invention is to provide an improved ionization detector comprising a double chamber structure with one of the chambers being the basic sensing chamber with porting being provided between the chambers to compensate for slow ambient changes. The sensing chamber is preferably ported to both the secondary chamber and the atmosphere outside of the chamber structure.
Still a further object of the present invention is to provide a simple means of adjusting the voltages available from the ionization chamber. Actually, one adjustable electrode can be used in each chamber if it is a two chamber structure.
Another object of the present invention is to provide an ionization chamber structure that comprises baffles for directing the air to be sensed and that further comprises an electrostatic screen for the ionization chamber or chambers.
Still another object of the present invention is to provide a unique electronic circuit which will provide an inexpensive and reliable means for detecting the signal change which occurs in the ionization chamber.
A further object of the present invention is to provide a means for adjusting the decision level of the alarm circuit of this invention to allow for any desired sensitivity setting.
Still a further object of the present invention is to provide means associated with the circuitry for providing a visual indication of the condition of the ionization chamber structure.
Another object of the present invention is to provide a three chamber structure characterized by a built-in feedback path that regeneratively stabilizes the operating point of the device.
A further object of this invention is to provide a specially designed chamber construction inluding electrodes shaped that enhance the efficiency of the chamber and reduce ion recombination.